Thermoplastic polyurethane (TPU) polymers are normally made by reacting a polyisocyanate with a hydroxy terminated intermediate (known as a polyol) and a short chain diol chain extender. The hydroxy terminated intermediate (polyol) forms the soft segment of the polymer, while the polyisocyanate and chain extender form the hard segment of the polymer. The hardness of the TPU polymer can be controlled by the amount of hard and soft segments in a particular TPU.
U.S. Pat. No. 4,376,834 discloses a rigid TPU which is made using small amounts, and preferably no polyols. The rigid TPU is made by reacting a polyisocyanate with a short chain diol chain extender. Rigid TPUs of this type are often referred to as engineering resins and can be used in structural applications, such as automotive parts, and the like.
The rigid TPUs of U.S. Pat. No. 4,376,834 have also found use in fiber composites. The normal method to produce such composites is by a process known as pultrusion. In the pultrusion process, fibers are pulled through a polymer melt to coat the fibers with the polymer. It is desirable to have the polymer melt have the lowest viscosity possible to enhance the “wetting” of the fibers by the polymer. Good wetting by the polymer enhances the properties of the composite.
The viscosity of the polymer melt can be reduced by increasing the temperature of the polymer melt. TPU polymers tend to depolymerize when heated to temperatures necessary to lower their viscosity enough to achieve good wetting of the fibers in a pultrusion process. When a TPU has been subject to depolymerization, it will have reduced physical properties, such as modulus, impact strength and molecular weight.
It has been suggested that this problem can be solved by adding a TPU catalyst to the TPU reactants when the TPU is first made. The catalyst would speed the depolymerization in the melt, thus reducing the melt viscosity, but be available to repolymerize the TPU as the melt is cooled.
In processes, such as the one-shot polymerization process to make highly rigid TPU, adding catalyst to the polymerization process can create other problems. Highly rigid TPU made by reacting a polyisocyanate with a short chain diol, the reaction is very exothermic and is difficult to remove the excess heat in the one-shot process. The adding of catalyst to this process increases the reaction rate and makes the problem more pronounced.
It would be beneficial to have a method of adding a TPU catalyst to an already prepared highly rigid TPU, so the catalyst would be available in subsequent processing to depolymerize and repolymerize the TPU in a quick manner.